1. Field of the Invention
The present invention relates to a wet-type plate-making machine for producing printing negatives, or, in particular, to a developing unit and a fixing unit for drying a master sheet developed by a developer.
2. Description of the Related Art
As is well known, a wet-type plate-making machine for producing printing negatives includes a developing unit in which a developer containing a toner is injected from a spray nozzle onto a master sheet formed with a latent image so that the latent image is developed and a toner image is formed on the master sheet. A developing tank of this developing unit is generally equipped with a concentration detection unit for detecting the toner concentration of the developer in circulation and a liquid amount detection switch for detecting the liquid level of the developer contained in the developing tank. A toner refill pump is controlled by a signal from the concentration detection unit to thereby maintain the toner concentration in the developing tank within a predetermined range.
In this conventional wet-type plate-making machine, the developer pump is controlled by the detection signal from the liquid amount detection switch, so that a substantially constant liquid level is maintained in the developing tank.
In this local control of the developing unit by the concentration detection unit and the liquid amount detection switch, however, an excessive liquid toner may be added in the developing tank to such an extent that the developer overflows the developing tank due to a shortage of the toner concentration in spite of the fact that the liquid level of the developer in the developing tank is almost at an upper limit.
Also, in the case where a toner concentration higher than a specified range is indicated by the concentration detection unit for detecting the shortage or overage of the toner concentration, the developer refill pump is started and the developer is added to the developing tank thereby to reduce the developer concentration. The refilling operation of the developer refill pump, however, is continued even in the absence of the developer in the developer bottle, in which case the developer refill pump undesirably runs under no load. For preventing the no-load operation of the developer refill pump, a special residual amount detector is required for monitoring the residual amount in the developer bottle.
In the case where the concentration detection unit transmits a detection signal indicating that the toner concentration below is a specified range, the toner refill pump is started to refill the toner in the developing tank and thus increase the developer concentration. The operation of this toner refill pump, however, is continued even when no liquid toner remains in the liquid toner bottle, in which case the toner refill pump is operated under no load. Also for preventing the no-load operation of the toner refill pump, a special residual amount detector is required for monitoring the residual amount of the liquid toner in the liquid toner bottle.
The conventional concentration detection unit includes a light-emitting device and a photo-detector on either side of a light transmitting cell such as a transparent glass tube through which the circulating developer flows. The amount of light transmitted from the light-emitting device through the light transmitting cell is received by the photo-detector thereby to detect the toner concentration of the developer flowing through the light transmitting cell. Thus, it is determined that the toner concentration is high when the intensity of the light received by the photo-detector is low, and low when the intensity of the light received by the photo-detector is high.
In this control of the developing unit by the concentration detection unit, however, it is often determined that the toner concentration of the developer in the developing tank is high due to aging of the developing unit in operation.
More specifically, in an actual wet-type plate-making machine, the surface of the light-transmitting cell, the luminous surface of the light-emitting device or the light-receiving surface of the photo-detector are liable to be contaminated by internal dust and dirt of the machine after the machine has been in operation for some time. For the same luminous intensity of the light-emitting device, therefore, the amount of the light reaching the photo-detector is reduced by the contamination, with the frequent result that a toner concentration higher than the actual value is detected.
For this reason, it has been normal practice to clean the concentration detection unit at the time of maintenance work on the wet-type plate-making machine at regular or irregular time intervals in order to secure the same received light intensity of the photo-detector for a specified luminous intensity of the light-emitting device.
This maintenance work, however, is both troublesome and time-consuming, and strict execution of this work requires removal of the light-transmitting cell from the developing tank and cleaning the inner wall surface thereof. This leads to the problem of an increased maintenance cost.
In a wet-type plate-making machine for producing a printing negative, a master sheet developed with a toner image by a developer is supplied to a fixing unit called an "oven heater" to fix the toner image and dry the master sheet.
A ceramic heater is often used as a heat source for the conventional fixing unit of "oven heater" type. In this fixing unit, the master sheet supplied thereto is heated and dried by the radiated heat of the ceramic heater thereby to fix a toner image. In the process, a considerable amount of heat is required for drying the master sheet rapidly.
The rate at which the master sheet is dried by the radiation heat, however, has its limit. When activating the plate-making machine in a low ambient temperature or during the cold season, the master sheet is not dried sufficiently and may be delivered without being fixed sufficiently.
In other words, once the master sheet is delivered without being sufficiently dried, the residual moisture causes the master sheets to attach to each other, often deteriorating the quality of the completed toner image. In an extreme case, the master sheets are difficult to separate from each other, or other various adverse effects are caused.